Injection moulding may be used to make a wide variety of articles including articles having relatively complex shapes and a range of sizes. Injection moulding is, for instance, suited to the manufacture of articles used as caps and closures for food and drink applications, such as for bottles containing carbonated or non-carbonated drinks, or for non-food applications like containers for cosmetics and pharmaceuticals.
Injection moulding is a moulding process in which a polymer is melted and then filled into a mould by injection. During initial injection, high pressure is used and the polymer melt is compressed. Thus, upon injection into the mould the polymer melt initially expands or “relaxes” to fill the mould. The mould, however, is at a lower temperature than the polymer melt, and therefore as the polymer melt cools, shrinkage tends to occur. To compensate for this effect, back pressure is applied. Thereafter the polymer melt is cooled further to enable the moulded article to be removed from the mould without causing deformation.
An important property of an injection moulded article is its stress crack resistance. It will be appreciated that the injection moulded articles of the invention should not exhibit brittle failure and should therefore possess a high stress crack resistance. An increase in stress cracking resistance is however, generally associated with decreases in tensile strength, e.g. in tensile modulus. It will also be appreciated that injection moulded articles are preferably stiff. This decrease in tensile modulus is particularly marked for HDPE. The present inventors sought new HDPEs, developed in particular for the cap and closure market, which posses improved stress cracking resistance and high tensile modulus. To add to the challenge however, these improvements must not be at the expense of processability of the polymer or the appearance of any article formed. Processability must be maintained or even improved to meet customer needs. Injection moulded articles are produced rapidly and any reduction in processability can increase cycle times and hence reduce process efficiency.
The present inventors have found that if HDPEs possess a certain relationship of molecular weight properties at melt flow rates appropriate for injection moulding, high stress crack resistance and tensile strength can be achieved. In particular, the present invention describes a multimodal HDPE polymer with tailored molecular weight that results in improved FNCT without reduction in tensile modulus. Our FNCT is clearly improved over a selection of comparable commercial polymer grades. In addition, caps or closures produced using this polymer have better aspect, specifically in terms of lower high-tips and less angel-hair.
When a cap or closure is formed in the injection moulding process, there is normally a small defect at the point of injection on top of the cap. This defect is a slightly raised portion on the top of the cap and is called a high tip. Whilst it is difficult to observe with the naked eye, the high tip can usually be felt on the top of most caps and closures. The polymers of the present invention allow this high tip to be minimised in size.
We attach hereto as FIGS. 1 and 2, pictures of a high tip (FIG. 2) which is generally unacceptably extended, and a low “high tip” (typically one that is less than 0.5 mm in height—FIG. 1) which is the target in the industry.
Moreover, when the injection moulding process is complete, a further problem which can occur is the formation of angel hair. Angel hair is fibre like strands of polymer that form on the top of the cap at the injection point as the cap is moved away from the injection moulding nozzle in the continuous injection moulding process. If the injection moulding process and the polymer used is not ideally suited to the injection moulding process, the polymer melt can stretch to form these fibre like hairs. The polymers of the invention also minimise formation of such hair. FIG. 3 shows the formation of angel hair on a cap. Angel hairs can have serious consequences for further manipulation of the cap, e.g. printing thereon and upon its appearance.
The invention relies on the use of polymers which have particular molecular weight distribution through a comparison on their Mz, Mn and Mw values. The present inventors have found that a particular relationship of Mz, Mw and Mn gives rise to polymers with advantageous properties. In particular therefore, the ratio of Mz/Mw must be low compared to the ratio of Mw/Mn. The relationship in claim 1 defines polymers that have a less pronounced high molecular weight tail. This does not prevent the polymers possessing a relatively broad molecular weight distribution Mw/Mn however.
Without wishing to be limited by theory, it may be that the problem of angel hair is exacerabated by the presence of high molecular weight chains within the polymer. It may be that because the polymers of the invention have a less pronounced high molecular weight tail that the polymers offer benefits in terms of minimising angel hair. Also, the inventors suggest that high levels of Mz/Mw may result in the formation of larger “high tips” on caps Our polymers may therefore enable the formation of lower “high tip”.
For avoidance of doubt, these high tips are so small that cutting them off is impractical. Also, caps are produced rapidly in high numbers and the cost of even attempting a cutting process on a plurality of caps would be prohibitive.
The advantageous properties of the HDPE of the invention can also be achieved without loss of processability. Again, the relationship between the high Mw and low Mw chains within the polymer of the invention means that the processability of the polymers of the invention is excellent.
In EP-A-1940942, HDPE compositions are described primarily for blow moulding applications. The compositions comprise a blend of unimodal HDPE and a high Mw unimodal polymer to thus form a bimodal composition. The polymers do not satisfy the ratio in claim 1 however.
The present inventors have compared the polymer of the invention to a broad range of commercial injection moulding grades of comparable tensile modulus to show that the relationship in claim 1 is not one which can be found in commercial polymers and is one which yields the advantageous properties highlighted above.